1. Field of the Invention
This invention relates to composite materials employed in the fabrication of nonmetallic expansion joints, in particular those serving as flue duct seals in low pressure high temperature gas service installations.
2. Description of the Prior Art
Nonmetallic expansion joints are used in both liquid service and gas service installations.
Liquid service expansion joints must be capable of operating under wide ranges of pressure and temperature, e.g., pressures ranging from full vacuum to 150 p.s.i. and higher, and temperatures ranging from below 0.degree. F. to 300-400.degree. F. Such expansion joints are predominantly manufactured from single ply materials supplied in roll form as "roll goods", e.g., rubber and woven fabrics. Expansion joint manufacturers typically employ molding and hand lay up techniques to produce composites of such materials with relatively thick cross sections on the order of 0.5 inches. The resulting expansion joints are very stiff, thereby requiring significant forces to generate any movement or flexing.
In contrast to the foregoing, gas service expansion joints, more commonly referred to as "flue duct seals", are designed to operate under relatively low pressure conditions, typically .+-.5 p.s.i., and at temperatures ranging from below 0.degree. F. to 1400.degree. F. and higher. The composite materials used in the manufacture of flue duct seals can have relatively thin cross sections on the order of 0.006 to 0.25 inches, typically including a single ply of woven fabric combined with both rubbers and perfluoroplastics. Thicker constructions include additional woven fabric lies. Such composite materials are usually manufactured by coating or laminating techniques and are also supplied as roll goods to expansion joint manufacturers. Ideally, these composite materials should be inherently flexible, and capable of readily elongating under relatively low stress conditions.
The woven fabrics used as the load bearing components of expansion joint composites are commonly "square weaves". Such fabrics are high modulus materials that do not readily stretch or elongate when stressed in the direction of their warp or fill yarns. However, the same materials are capable of readily stretching if they are arranged with their yarns on a bias with respect to the direction of stress. Thus, in situations where the ability to readily stretch or elongate is critical, as is often the case in the expansion joint industry, expansion joint manufacturers have resorted to relatively complex and labor intensive fabrication techniques in order to achieve a bias orientation of the conventional composite materials currently available on the open market.
During at least the last 20-30 years, this has been accomplished by cutting the conventional composite materials into discrete segments which are reoriented by 45.degree. and then spliced back together to form so called "belts". The belts are then fabricated into expansion joints, with the warp and fill yarns of the load bearing components arranged on a bias with respect to the expected directions of major stress.
This procedure was reasonably suited to the earlier composite materials, which typically comprised single plies of woven fiberglass coated with rubber. Splicing was easily achieved at relatively low temperatures. However, with the enactment of more aggressive air pollution legislation in the late 1960's and early 1970's, there arose a need for more sophisticated composite materials, with greater resistance to chemical attack and with a greater ability to span wider gaps between equipment components.
To meet these demands, more complex rubber composites came on the market, with fluoroelastomer coatings and multiple layer constructions containing two or more woven fabric plies. These more complex composite constructions could not readily be subdivided and spliced back together to achieve a bias orientation of the woven fabric load bearing components. Thus, bias orientation remained largely limited to the fabrication of expansion joints from the earlier composite materials.
In the 1980's and 1990's, composite expansion joint materials combining woven fabric load bearing components with perfluoroplastics such as polytetrafluoroethylene (PTFE) began acquiring a meaningful market share. However, splicing of these materials involved new fabrication procedures requiring the use of irons heated to elevated temperatures on the order of 725.degree. F. Such procedures were unfamiliar to the expansion joint industry. Thus, very little bias production of expansion joints employing these PTFE based composites took place, and then only by cutting the materials into segments which were then reoriented on a bias and spliced back together, as was done earlier with the simple rubber based products.
In recent years, a significant and increasing amount of new power generation is being based worldwide on gas and diesel turbines. This equipment operates at much higher temperatures, with an attendant increase in thermally induced movement between equipment components. This has prompted the development of even more complex fluoropolymer based composite materials, typically comprising PTFE coated woven fabric substrates combined with sophisticated corrosion barriers, thermal barriers and other associated components in multi layer laminated composites.
To date, use of these more sophisticated composite materials in bias oriented configurations has been limited because conventional techniques for doing so dictate that the entire composite must be cut through in order to provide segments which can be reoriented and reassembled by splicing. The procedures for splicing the individual composite plies in a manner that retains their continuity are exceedingly difficult and often unreliable. Failure to properly splice the corrosion and/or thermal barrier components can result in the fabrication of flue duct seals which fail prematurely in service.
Use of these materials without arranging their fabric substrates on a bias has led to other problems, particularly in flue duct seals operating at very high temperatures, where movement between equipment components is most pronounced. For example, the inability of such composite materials to readily elongate or stretch can lead to the formation of severe creases and/or wrinkles. When wrinkles develop, the resulting folds lose the cooling effect of ambient air. This in turn can produce "hot spots" or burned areas that will require replacement of the expansion joint within a very short period.
Accordingly, it is an object of the present invention to provide a flexible composite expansion joint material having as its load bearing component a fluoropolymer containing woven fabric substrate which has been segmented and reoriented into a bias configuration, without attendant disruption or compromise in the continuity and integrity of associated fluid corrosion barrier and/or thermal barrier components.
A further objective of the present invention is the provision of an improved method for producing the aforesaid expansion joint material as roll goods for use in the fabrication of flue duct seals.